Intaglio printing refers to a printing method used in particular in the field of printing security documents.
In the intaglio printing process, a rotating engraved steel cylinder carrying a plate engraved with a pattern or image to be printed is supplied with ink by one or by a plurality of selective inking cylinder(s) (or chablon cylinders), each selective inking cylinder being inked in at least one corresponding colour to form multi-shade visible features.
Furthermore, the intaglio printing process involves a wiping off of any ink excess present on the surface of the intaglio printing device. The wiping off process is carried out using a paper or a tissue wiping system (“calico”), or a polymeric roll wiping system (“wiping cylinder”). Because of the amount of waste material and of the printing speed on an industrial printing press, the wiping off with a rotating wiping cylinder is increasingly preferred; wiping off with paper or tissue is practically not used anymore on an industrial printing press. The wiping cylinder is in turn cleaned in a bath comprising a solvent or an aqueous solution; or the wiping cylinder is cleaned with a solution spray; optionally brushes or Scotch-Brite™ material may also be used additionally. Because of environmental concerns and regulation on volatile organic compounds, cleaning of the wiping cylinder with solvent is practically not used anymore. Typically the washing solution used to clean the wiping cylinder is a basic aqueous solution comprising caustic soda and surfactant such as e.g. sulfated castor oil (SCO).
Subsequently, the plate is brought into contact with a substrate, e.g. with a paper, a composite or a plastic material in sheet form or web form, and the ink is transferred under pressure from the engravings of the intaglio printing device onto the substrate to be printed forming a thick printing pattern on the substrate.
FIG. 2a shows a schematic representation of an intaglio printing press. In the FIG. 2a, the represented intaglio printing press operates with a direct inking process.
In FIG. 2a, the cylinder (100) represents the cylinder carrying the intaglio plate. Each of the devices (110) and (111), (112) and (113), and (114) and (115) represent a fountain and an ink train for three separate intaglio inks. The fountain functions as a reservoir of intaglio ink. Each intaglio ink is supplied from its individual ink fountain well. Each ink train consists in a series of rollers. The ink train facilitates the distribution and the transfer of the ink from the fountain to the cylinder carrying the intaglio plate. Within each ink train, an inking roller ((110), (112) or (114)) coated with a polymeric material transfers the corresponding intaglio ink to a chablon ((111), (113) or 115)). A chablon of an intaglio printing press is a sheet of material comprising some relief on which the ink is deposited; the chablon is located between the roller of each ink train that is located next to the intaglio plate, and the intaglio plate on the plate cylinder (100). The ink is forced from the relief of the chablon into the engravings of the intaglio plate.
The intaglio ink is transferred from the chablons into the engravings of the intaglio plate; however some ink excess may also be transferred onto the non-engraved surface of the plate. The ink excess on the surface of the plate cylinder is removed either by cleaning the cylinder carrying the intaglio plate (100) with a wiping cylinder (120) and a cleaning solution; or, alternatively, the ink excess from the surface of the cylinder is removed from the cylinder (100) by wiping with a paper or a tissue (“calico”). From the plate cylinder (100), the ink is transferred to the substrate to be printed (130) under high pressure, to form the printed intaglio features (180). Typically, a pressure of several tens to several hundreds of bars is applied during an intaglio printing process. A counter-pressure cylinder (170) is located on the opposite side of the substrate.
The ink fountains, the rollers (110), (112) and (114) of the ink trains, and the plate cylinder (100) are usually equipped with temperature control system. Typical settings for intaglio printing press involve the ink fountain being kept at 20° C. while the plate cylinder is maintained at a temperature around 60° C. to 80° C.
The printing press shown in FIG. 2a may be used with oxidatively drying intaglio inks. In this case, the non-metallic parts of the printing press, in particular the rollers (110), (112) and (114) of the ink trains, and the corresponding chablons (111), (113) and (115), are typically made of rubber, preferably PUR rubber. The wiping cylinder (120) is typically made of polyvinyl chloride (PVC) or rubber.
When oxidatively curable intaglio inks are used to print an intaglio feature, the intaglio ink hardening starts immediately after the removal of the printed substrate from the intaglio printing cylinder (100). The oxidative curing may however typically be accelerated by a thermal treatment of the printed feature. The intaglio printing press shown in the FIG. 2a is equipped with a heat source (140), e.g. a hot air drier. Such a heat source is typically used with oxidatively curable intaglio inks to accelerate the ink hardening.
In an alternative intaglio process called Orlof process after the name of its inventor and schematically represented in the case of an intaglio press in FIG. 3a, the intaglio ink is transferred from the ink trains and the chablons to a collecting cylinder (160), called the “blanket”, which, in turn, transfers the inks to the cylinder (100) carrying the intaglio plate; finally the inks are printed from the plate cylinder (100) onto the substrate (130) to form the printed intaglio features (180). The process is called indirect inking process. The indirect intaglio inking process brings in particular two benefits: reduced ink consumption and new design possibilities; in particular, the design possibilities benefit from the extremely precise inking and colour splits.
Intaglio printing presses have been described in detail e.g. in EP 0 091 709 A1, EP 0 406 157 A1, EP 0 563 007 A1, EP 0 873 866 A1, EP 1 602 482 A1 or US 2010 0 139 511 A1.
Intaglio printing delivers the most consistent and high quality printing of fine lines. It is the printing technology to be chosen for generating fine designs in the field of security documents, in particular banknotes and stamps.
One of the distinguishing features of the intaglio printing process is that the intaglio relief may be varied from a few micrometers to several tens of micrometers by using correspondingly shallow or deep recesses on the intaglio printing device. This ability to vary the intaglio relief is characteristic of the intaglio printing process and is used to confer tactility to the printed document. The intaglio relief results from the intaglio ink layer thickness which is emphasized by the embossing of the substrate produced by the pressure during the ink transfer. The tactility resulting from intaglio printing gives the banknotes their typical and recognizable touch feeling.
Due to the intaglio relief, intaglio printed devices are particularly prone to potential problems such as e.g. set-off and/or blocking problems.
“Set-off”, which is the transfer of ink from one printed sheet to the back side of the next following printed sheet in the stack, or to the back of an endless sheet in a web, is a problem that may be encountered with any industrial printing process, in particular gravure and intaglio printing methods: the pronounced relief of gravure printing methods may accentuate the problem of set-off. Thus, interleave sheets between adjoining sheets have been used to solve this issue; however interleave sheets add additional material and handling costs to the printing process, reduce the maximal printing speed and need to be removed before the next printing step. With the state of the art oxidatively drying intaglio inks, the set-off issues have been reduced through the optimization of the ink formulation; however, deep engraving features may still result in undesirable set-off.
“Blocking” in the stack or in the reel, which is the sticking adhesion of one printed sheet to the back side of the next printed sheet in the stack, or to the back of an endless sheet in a web, is a fault that results from the weight or the pressure in the stack or in the web, and from the affinity of the printed ink for the backside of the next sheet or the backside of the endless sheet in the web; the problem may be encountered with any industrial printing process, in particular with gravure printing methods. Tactile features or thick printed layers printed with gravure printing process may in particular favour the occurrence of blocking since the weight of the stack or the web is concentrated on the tactile features thus producing an increased pressure on these thick-layered embossed areas.
Alternatively the set-off and blocking issues have been solved by the development of new UV-VIS-curable intaglio inks, as disclosed for instance in EP-1 260 563 A1, EP-0 432 093 A1 or US 2009/0145314 A1: UV-VIS-irradiation leads to very fast ink curing, in particular surface curing, and hence reduces drastically the set-off issue. Intaglio printing with UV-VIS-curable intaglio inks has proven to be particularly useful for difficult engraving design, for instance for deep engraving (thick ink layers): the instant surface curing of UV-VIS-curable inks prevents the set-off issues.
FIG. 2b shows a schematic representation of an intaglio printing press similar to the printing press of FIG. 2a; however, the thermal source (140) of FIG. 2a has been substituted with a source of electromagnetic radiation (150) for the UV-VIS curing of a UV-curable intaglio ink.
Printing presses, e.g. intaglio printing presses, usually comprise metallic elements and elements made of polymeric material. In the case of an intaglio printing press, the polymeric material elements include in particular the rollers of the ink trains, the chablons and the wiping cylinder.
The polymeric material used for these polymeric material elements consists for instance of rubber, polyurethane rubber (PUR rubber), silicone rubber, polyvinyl chloride (PVC), polyfluoroethylene (Teflon), ethylene propylene diene monomer (EPDM). In order to increase the lifetime of the polymeric material elements, the polymeric material is selected according to the type of ink to be used; in particular, the polymeric material of the elements which are continuously in contact with the inks, such as e.g. the rollers and the chablons, is selected such as to ensure an optimum lifetime of these elements. For instance, for oxidatively drying inks, PUR rubber is preferred; on the other hand, for UV-curable inks, EPDM is preferred to PUR rubber because the monomer and/or oligomer components of UV-curable inks tend to cause swelling and/or stickiness of PUR rubber. Examples of material used for the polymeric elements are disclosed e.g. in WO 2009/013 169 A1; WO 2003/066 759 A1; Rubber rollers in today's printing processes, T. L. Traeger, Rubber World, Oct. 1, 1999; Böttcher Systems in bottcher.com).
Thus in FIG. 2b, the non-metallic parts of the printing press are preferably made of material compatible with UV-curable intaglio inks; in particular, the rollers (110), (112) and (114) of the ink trains, and the corresponding chablons (111), (113) and (115), are preferably made of material compatible with UV-curable inks, such as e.g. EPDM material. The wiping cylinder (120) is typically made of polyvinyl chloride (PVC) or rubber.
In the Orlof process, as used in intaglio printing or in offset, the collecting cylinder is made of a woven fabric material coated with rubber. Examples of blanket material comprise e.g. polyurethane rubber (PUR rubber), acrylonitrile butadiene rubber (NBR); examples are given e.g. in U.S. Pat. No. 5,264,289 A, WO 2007/062271 A and JP 2011/173376-7 A.
In order to increase the lifetime of the blanket, the fabric material of the blanket is selected according the ink composition to be used. Nevertheless, some materials, e.g. acrylonitrile butadiene rubber (NBR), silicone rubber, show excellent resistance to both oxidatively drying inks and UV-curable inks and are thus used preferably (see e.g. U.S. Pat. No. 5,264,289 A).
Curing of oxidatively drying inks is typically a slow process which results in a higher tendency of the oxidative inks, as compared to UV-VIS-curable inks, to produce set-off. Furthermore, the relatively slow drying process of the oxidatively drying inks results also in slower printing process as compared to the UV-VIS-curing process.
Furthermore, thick transparent colourless indicia printed with oxidatively drying inks tend to turn yellow upon aging. Thus for the intaglio printing of transparent thick ink layers, UV-VIS-curable inks are preferred.
Due to their fast or almost immediate curing, intaglio printing with UV-VIS-curable inks allows reducing the time between printing and handling of the printed substrates, and increasing the number of stacked sheets per pile. The presence of volatile organic compounds can be avoided with intaglio UV-VIS-curable inks. UV-VIS-curable intaglio inks are also significantly more stable on the printing press than oxidatively drying inks.
A shortcoming of the UV-VIS-curable intaglio inks is their significantly higher cost which contributes very much to their marginal market penetration.
Some attempts to combine the advantages of both technologies in one ink have been disclosed. For instance, WO 2011/046083 A1, JP 2009/227702 A and JP 2011/068748 A disclose an intaglio ink composition comprising a UV-VIS-curable composition, an oxidation-curable composition, a photoinitiator, an oxidation polymerization catalyst and a pigment. WO 2003/066759 A1 discloses a similar composition wherein the UV-VIS-curable component of the intaglio composition is water-soluble.
The high pressure applied during the intaglio printing process may also serve as a means for sealing the surface of a substrate, e.g. paper, even in the non-intaglio printed areas; thus intaglio printing contributes to preserve a document against soiling. EP 2 065 187 B1 KBA-NotaSys discloses a process using transparent or semi-transparent intaglio inks applied on at least 80% (the percentage is based on the total surface of one side of the security paper) of the surface of the security papers in order to prevent the soiling of the substrate. Thus EP 2 065 187 B1 discloses a process principally directed at varnishing the substrate than at printing a particular intaglio security feature. In EP 2 065 187 B1, no details concerning the composition of the used intaglio inks are given.
Sequential intaglio printing with an oxidatively drying ink and UV-VIS-curable ink has been disclosed e.g. in DE 4 444 034 A1; the disclosed method, however, is a two-step printing process which requires the modification of the commonly used intaglio printing equipment.
It would be highly desirable to create specific intaglio designs such as for instance, but not limited to, the juxtaposition of a highly pigmented ink layer and a thick transparent intaglio ink layer (as for instance for tactile blind/low-vision features) in one printing step on one intaglio cylinder, in an improved way.
None of the above prior art documents discloses the simultaneous use of oxidative-drying and of UV-VIS-curable intaglio inks on one cylinder to produce specific intaglio security features.